1. Field of the Invention
The present invention relates to pneumatic tires having an improved tread structure.
2. Related Art Statement
For example, pneumatic tires as described in Japanese patent application Laid-open No. 63-93,604 are formerly known, in which their tread structure is improved. Such tires are pneumatic tires having a cap/base structure in which a tread is constituted by an outer tread portion which is arranged on a radially outer side of the tire and made of a single kind of a rubber and an inner tread portion which is arranged radially inside a surface of a wearing indicator of the tread and also on a radially inner side of the tire and made of plural kinds of rubbers. The inner tread portion is divided into at least three zones in an axial direction of the tire, that is, a central zone spread over tire equator and side zones continuously positioned on the opposite sides of the central zone. The hardness of the tread rubber in the central zone of the inner tread portion is made smaller than that of the tread rubber in the side zones and that of the tread rubber in the outer tread portion, and hardness of the tread rubber in the inner tread portion is increased as it goes from the central zone to the side zones. Further, hardness of the tread rubber of that portion of the inner tread which is located on the axially outermost portion of the tire is not greater than that of the tread rubber in the outer tread portion. When the hardness of the rubber of the tread is decreased as it goes to the radially inner side and to the axially inner side in the above manner, anti-vibration riding comfortability can be improved without deteriorating wear resistance.
The tread rubber, that is, the rubber of the inner and outer tread portions is all made of a non-foam rubber in the above-mentioned tire. When compression forces are applied to such a non-foam rubber, its volume is not almost reduced. Thus, even when the hardness of the rubber of a part of the tread, that is, the inner tread portion, is decreased as mentioned above, a compression modulus of the entire tread does not almost decrease in actuality. As a result, anti-vibration riding comfortability can merely be improved to some extent, but no sufficient effect cannot be obtained during actual use. In addition, the tensile modulus of an ordinary non-foam rubber is reduced corresponding to reduction in the rubber hardness as mentioned above.